When W32.Welchia.Worm is executed, it performs the following actions:
- Copies itself to:
%System%\Wins\Dllhost.exe
NOTE: %System% is a variable. The worm locates the System folder and copies itself to that location. By default, this is C:\Winnt\System32 (Windows 2000) or C:\Windows\System32 (Windows XP).
- Makes a copy of %System%\Dllcache\Tftpd.exe as %System%\Wins\svchost.exe.
NOTE: Tftpd is a legitimate program, which is not malicious, and therefore Symantec antivirus products do not detect it.
- Adds the subkeys:
RpcPatch
and:
RpcTftpd
to the registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services
- Creates the following services:
Service Name: RpcTftpd
Service Display Name: Network Connections Sharing
Service Binary: %System%\wins\svchost.exe
This service will be set to start manually.
Service Name: RpcPatch
Service Display Name: WINS Client
Service Binary: %System%\wins\dllhost.exe
This service will be set to start automatically.
- Ends the process, Msblast, and deletes the %System%\msblast.exe file, which W32.Blaster.Worm drops.
- Selects the victim IP address in two different ways: The worm uses either A.B.0.0 from the infected machine's IP of A.B.C.D and counts up, or it will construct a random IP address based on some hard-coded addresses.
After selecting the start address, the worm counts up through a range of Class B-sized networks; for example, if the worm starts at A.B.0.0, it will count up to at least A.B.255.255.
- Sends an ICMP echo request, or PING, to check whether the constructed IP address is an active machine on the network.
- Once the worm identifies a machine as being active on the network, it will either send data to TCP port 135, which exploits the DCOM RPC vulnerability, or it will send data to TCP port 80 to exploit the WebDav vulnerability.
- Creates a remote shell on the vulnerable host, which reconnects to the attacking computer on a random TCP port, between 666 and 765, to receive instructions.
Note: In the vast majority of the cases, the port is 707, because of the way the worm-threading model interacts with the implementation of the Windows C runtime .dll.
- Launches the TFTP server on the attacking machine and instructs the victim machine to connect and download Dllhost.exe and Svchost.exe from the attacking machine. If the %System%\dllcache\tftpd.exe file exists, the worm may not download svchost.exe.
- Checks the computer's operating system version, Service Pack number, and System Locale. It also attempts to connect to Microsoft's Windows Update and download the appropriate DCOM RPC vulnerability patch.
- Once the update has been downloaded and executed, the worm restarts the computer so that the patch is installed.
- Checks the computer's system date. If the year is 2004, the worm will disable and remove itself as follows:
- Deletes the file %System%\Wins\Dllhost.exe
- Deletes the services, RpcPatch and RpcTftpd, and removes the associated registry keys:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RpcPatch
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RpcTftpd
The worm does not delete the file, %System%\Wins\Svchost.exe, which is a nonmalicious tftp server.
Notes:
- The worm activates its removal routine only if the worm is started in the year 2004. If the worm has been running continuously since 2003, it will not remove itself after January 1, 2004 unless you manually restart the computer or worm.
- The W32.Welchia.Worm removal tool will still function normally in 2004.
Intruder Alert
On August 19, 2003, Symantec released
Intruder Alert 3.6 W32_Welchia_Worm Policy.
Norton Internet Security/Norton Internet Security Professional
On August 20, 2003, Symantec released IDS signatures via LiveUpdate to detect W32.Welchia.Worm activity.
Symantec Client Security
On August 20, 2003, Symantec released IDS signatures via LiveUpdate to detect W32.Welchia.Worm activity.
Symantec ManHunt
- Symantec ManHunt Protocol Anomaly Detection technology detects the activity associated with this exploit as "Portsweep." Although ManHunt can detect activity associated with this exploit with the Protocol Anomaly Detection technology, you can use the "Microsoft DCOM RPC Buffer Overflow" custom signature, released in Security Update 4, to precisely identify the exploit being sent.
- Security Update 7 has been released to provide signatures specific to W32.Welchia.Worm to include the detection of more W32.Welchia.Worm attributes.
Symantec Gateway Security
- On August 18, 2003, Symantec released an update for Symantec Gateway Security 1.0.
- Symantec's full application inspection firewall technology protects against this Microsoft vulnerability, blocking all the above listed TCP ports by default. For maximum security, third-generation, full application inspection technology intelligently blocks the tunneling of DCOM traffic over HTTP channels; thus, providing an extra layer of protection not readily available on most common network filtering firewalls.
Symantec Host IDS
On August 19, 2003, Symantec released an update for Symantec Host IDS 4.1.
Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":
